[PB-30] Speed up start of benchmark runs
[PB-30] Speed up start of benchmark runs

This commit divides the transactions into two batches of (roughly)
equal size. The first batch is constructed before sending any
transactions, and the second batch in parallel to sending the
transactions.

This should halve the startup time, and not decrease the transaction
rate.

input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl